The vascular endothelium produces vasodilator substances that help regulate vascular tone at rest and during physiologic stress in addition to influencing growth and thrombogenesis in the vessel wall. At present, the best characterized of these factors are nitric oxide (NO) and prostacyclin. Endothelium derived hyperpolarizing factor (EDHF)and the role for carbon monoxide (CO) have not been well characterized. Study 1: Endothelial and vascular smooth muscle cells (VSMC) possess hemoxygenase (HO). This enzyme catalyzes the breakdown of heme to bilirubin, biliverdin and CO. CO, like NO, activates intracellular guanylate cyclase in VSMC leading to the generation of cyclic guanosine monophosphate (cGMP) which causes vasodilation. Platelet cGMP is also increased by CO. Tin protoporphyrin 9(SnPP9) is a potent inhibitors of HO and reduces smooth muscle cell relaxation and platelet inhibition in vitro by reducing CO production. This study is designed to investigate the contribution of CO to forearm vascular function in healthy volunteers by assessing endothelium-dependent and - independent vasodilation before and during an intra-arterial infusion of SnPP9 before and after inhibition of NO.Study 2: EDHF release is stimulated by receptor-dependent agonists such as acetylcholine and bradykinin (BK), and leads to hyperpolarization of the underlying smooth muscle cells probably by opening Ca2+-activated K+ channels. Indirect pharmacological evidence in animal tissues suggests that EDHF is a cytochrome P450-derived arachidonic acid metabolite, probably an epoxide. In human arteries, potassium (20-40 mM) inhibits endothelium - mediated hyperpolarization. We studied the effect of intra- arterial potassium (KCl)in inhibiting BK-mediated forearm vasodilation after inhibition of NO and prostacyclin synthesis in healthy volunteers. KCl (0.33 mmol/min) produced a 166% increase in forearm blood flow. There was a net reduction in dilation when KCl was co-infused with BK but not SNP (an endothelium- independent agonist)indicating that KCl inhibits EDHF. - Nitric oxide, endothelium, hemoxygenase, carbon monoxide, endothelium derived relaxing factor - Human Subjects